The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a technique, including a method and apparatus, for improving the deposition rate of refractory metal layers.
Deposition of refractory metals, such as tungsten, over a semiconductor substrate is a common step in the formation of some integrated circuit (IC) structures. For example, tungsten is commonly used to provide electrical contact to portions of a semiconductor substrate. These electrical contacts are usually provided through openings in an insulation layer, such as a silicon dioxide layer, formed over the substrate. One method used to form such contacts includes the chemical vapor deposition (CVD) of tungsten to fill the opening after an initial layer of titanium nitride has been deposited in the opening. As another example, tungsten is sometimes used to form metal lines over a semiconductor substrate.
One CVD technique that has been employed to deposit tungsten films in the semiconductor industry uses tungsten hexafluoride (WF.sub.6) and a hydrogen reducing agent, e.g., H.sub.2, as precursor gases. This technique includes two main steps: nucleation and bulk deposition. The nucleation step grows a thin layer of tungsten which acts as a growth site for subsequent film. In addition to WF.sub.6 and H.sub.2, the process gas used in the nucleation step of this technique includes silane (SiH.sub.4), and may also include nitrogen (N.sub.2) and argon. A bulk deposition step then is used to form the tungsten film. The bulk deposition gas is a mixture containing WF.sub.6, H.sub.2, N.sub.2, and Ar.
Advances in integrated circuit technology have lead to a scaling down of device dimensions and an increase in chip size and complexity. This has necessitated improved methods for deposition of refractory metals, particularly tungsten which has led to a constant endeavor to decrease the quantity of impurities, such as ethylene, deposited in the refractory metal layers. The aforementioned impurities may have deleterious effects on the refractory metal layer, depending upon the nature of the impurity and the quantity present therein. Over the past ten years, impurity control has been successful in substantially reducing impurities attributable to the ambient environment in which refractory metal layers are formed so that greater than 80% of all impurities now present are a direct result of the process. One such source is the contaminants present in the process gases employed to form refractory metal layers. As a result, many process gases are produced in purified form so that there is less than ten, 10, parts of contaminants for every one billion, 1,000,000,000 parts of process gas. Such purification greatly increases the cost of the process gas and, therefore, the cost of depositing a refractory metal layer.
What is needed, therefore, is an improved method for depositing refractory metal layers that lowers the cost of producing the same.